This invention relates to an illumination apparatus and, more particularly, to an illumination apparatus arranged to provide an illumination beam of arcuate cross-section so that it is applicable particularly to an alignment and exposure apparatus of mirror projection type, for transferring an integrated-circuit pattern of a photomask onto a semiconductor wafer.
Alignment and exposure apparatuses of the mirror projection type are well known. Conventionally, in such alignment and exposure apparatuses, the photomask (which hereinafter will be referred to simply as "mask") has been illuminated in accordance with a well-known critical illumination method wherein a light source is imaged on the surface of the mask. An example is disclosed in U.S. Pat. No. 4,294,538 issued Oct. 13, 1981. The present inventor has found that such critical illumination for illuminating the mask involves inconveniences as follows:
The alignment and exposure apparatus of mirror projection type requires an illumination area of arcuate shape. According to the critical illumination, images of the light source are formed at the points within the arcuate illumination area on the mask surface and these light source images are inclined concentrically with respect to the axis of the optical system. That is, the directions of the converging light rays which are going to be focused on each point in the arcuate illumination area on the mask surface would vary along the arcuate illumination area, particularly in the directions from the middle to the end portions of the arcuate area. FIG. 1 shows this inclination schematically. In this Figure, reference character O shows the optical axis, A shows the arcuate illumination area, and B shows the light source images. As viewed in this Figure, the above-mentioned directions of the converging light rays with respect to each of the light source images B are the directions perpendicular to and toward that light source image. As compared with such concentric orientation of the light source images, the fine pattern of the mask for preparing the integrated circuits or the like generally shows a definite directionality with respect to each of the X-direction and the Y-direction, both of which are contained in a plane perpendicular to the optical axis O. More particularly, the mask pattern has pattern elements extending generally in two orthogonal directions contained in the mask plane. Therefore, there occurs a deviation between the orientation of the light source images (or the directions of convergence as described above) and the directionality of the pattern of the mask, which deviation increases toward each of the ends of the arcuate illumination area. As a result, only a slight amount of defocus would cause distortion of the pattern transferred onto the wafer. This is particularly notable at the end portions of the arcuate illumination area.